Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within cells (see, e.g., Hardie and Hanks, The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif., 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases can be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these families (see, e.g., anks & Hunter, (1995), FASEB J. 9:576-596; Knighton et al., (1991), Science 253:407-414; Hiles et al., (1992), Cell 70:419-429; Kunz et al., (1993), Cell 73:585-596; Garcia-Bustos et al., (1994), EMBO J. 13:2352-2361).
JAK kinases (JAnus Kinases) are a family of cytoplasmic protein tyrosine kinases including JAK1, JAK2, JAK3 and TYK2. Each of the JAK kinases is selective for the receptors of certain cytokines, though multiple JAK kinases can be affected by particular cytokine or signaling pathways. Studies suggest that JAK3 associates with the common gamma (γc) chain of the various cytokine receptors. JAK3, in particular, selectively binds to receptors and is part of the cytokine signaling pathway for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. JAK1 interacts with, among others, the receptors for cytokines IL-2, IL-4, IL-7, IL-9 and IL-21, while JAK2 interacts with, among others, the receptors for IL-9 and TNF-α. Upon the binding of certain cytokines to their receptors (e.g., IL-2, IL-4, IL-7, IL-9, IL- and IL-21), receptor oligomerization occurs, resulting in the cytoplasmic tails of associated JAK kinases being brought into proximity and facilitating the trans-phosphorylation of tyrosine residues on the JAK kinase. This trans-phosphorylation results in the activation of the JAK kinase.
Phosphorylated JAK kinases bind various STAT (Signal Transducer and Activator of Transcription) proteins. STAT proteins, which are DNA binding proteins activated by phosphorylation of tyrosine residues, function both as signaling molecules and transcription factors and ultimately bind to specific DNA sequences present in the promoters of cytokine-responsive genes (Leonard et al., (2000), J. Allergy Clin. Immunol. 105:877-888). JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant (allograft) rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis, as well as in solid and hematologic malignancies such as leukemia and lymphomas. For a review of the pharmaceutical intervention of the JAK/STAT pathway see Frank, (1999), Mol. Med. 5:432:456 and Seidel et al., (2000), Oncogene 19:2645-2656.
As described above, JAK's are crucial components of diverse signal transduction pathways that govern important cellular functions, including cell survival, proliferation, differentiation and apoptosis. Interfering with JAK activity may lead to the loss of a vital signal transduction pathway, thereby disrupting normal cellular processes needed for cell survival. Therefore, it is important to selectively inhibit particular JAK's that are involved in various disease states.
JAK2 associates with cytokine receptors and is essential for signal transduction by mediating tyrosine phosphorylation. JAK2 activity is regulated by a series of interactions, beginning with binding to specific domains in receptors, suppression of activation by the pseudokinase domain, and the requirement for phosphorylation within the activation loop. Recent studies have implicated deregulation of JAK2 kinase activity by chromosomal translocations in hematopoietic tumors and mutations within the pseudokinase domain in a spectrum of myeloproliferative diseases (Ihle, J N et al., Current Opinion in Genetics and Development (2007) 17:1, 8-14.
In addition, JAK2 has been suggested to be involved in the upregulation of angiotensinogen promoter activity in hypertrophy and ischemia (Mascareno E, et al. (2000) Mol. Cell. Biochem., 212:171-175; and Mascareno E, et al. (2001) Circulation, 104:1). One particular tyrphostin, AG490, selectively inhibits JAK2 and has been proposed for treating cancer (Meydan N, et al. (1996) Nature, 379:645). Administration of tyrphostin AG490 has been suggested to afford cardioprotection to hearts subjected to ischemia/reperfusion (Mascareno E, et al. (2000) Mol. Cell. Biochem., 212:171 and Mascareno E, et al. (2001) Circulation, 104:1). TG101209, a JAK2-selective compound, has been recently shown to be effective in inhibiting mutations in myeloproliferative disorders (Pardanani A, et al. (2007) Leukemia, 1-11).
JAK3, in particular, has been implicated in a variety of biological processes. For example, the proliferation and survival of murine mast cells induced by IL-4 and IL-9 have been shown to be dependent on JAK3—and gamma chain—signaling (Suzuki et al., (2000), Blood 96:2172-2180). JAK3 also plays a crucial role in IgE receptor-mediated mast cell degranulation responses (Malaviya et al., (1999), Biochem. Biophys. Res. Commun. 257:807-813), and inhibition of JAK3 kinase has been shown to prevent type I hypersensitivity reactions, including anaphylaxis (Malaviya et al., (1999), J. Biol. Chem. 274:27028-27038). JAK3 inhibition has also been shown to result in immune suppression for allograft rejection (Kirken, (2001), Transpl. Proc. 33:3268-3270). JAK3 kinases have also been implicated in the mechanism involved in early and late stages of rheumatoid arthritis (Muller-Ladner et al., (2000), J. Immunol. 164:3894-3901); familial amyotrophic lateral sclerosis (Trieu et al., (2000), Biochem Biophys. Res. Commun. 267:22-25); leukemia (Sudbeck et al., (1999), Clin. Cancer Res. 5:1569-1582); mycosis fungoides, a form of T-cell lymphoma (Nielsen et al., (1997), Prac. Natl. Acad. Sci. USA 94:6764-6769); and abnormal cell growth (Yu et al., (1997), J. Immunol. 159:5206-5210; Catlett-Falcone et al., (1999), Immunity 10:105-115).
The JAK kinases, including JAK3, are abundantly expressed in primary leukemic cells from children with acute lymphoblastic leukemia, the most common form of childhood cancer, and studies have correlated STAT activation in certain cells with signals regulating apoptosis (Demoulin et al., (1996), Mol. Cell. Biol. 16:4710-6; Jurlander et al., (1997), Blood. 89:4146-52; Kaneko et al., (1997), Clin. Exp. Immun. 109:185-193; and Nakamura et al., (1996), J. Biol. Chem. 271:19483-8). They are also known to be important to lymphocyte differentiation, function and survival. JAK-3, in particular, plays an essential role in the function of lymphocytes, macrophages, and mast cells. Given the importance of this JAK kinase, compounds which modulate the JAK pathway, including those selective for JAK3, can be useful for treating diseases or conditions where the function of lymphocytes, macrophages, or mast cells is involved (Kudlacz et al., (2004) Am. J. Transplant 4:51-57; Changelian (2003) Science 302:875-878). Conditions in which targeting of the JAK pathway or modulation of the JAK kinases, particularly JAK3, are contemplated to be therapeutically useful include leukemia, lymphoma, transplant rejection (e.g., pancreas islet transplant rejection), bone marrow transplant applications (e.g., graft-versus-host disease), autoimmune diseases (e.g., RA, type I diabetes), and inflammation (e.g., asthma, allergic reactions). Conditions which can benefit for inhibition of JAK2 are discussed in greater detail below.
In view of the numerous conditions that are contemplated to benefit by treatment involving modulation of the JAK pathway, it is immediately apparent that new compounds that modulate JAK pathways and methods of using these compounds should provide substantial therapeutic benefits to a wide variety of patients.